The present invention relates to the art of bi-directional printing. It finds particular application in conjunction with ink-jet printers and other similar printers or image-rendering devices with scanning or moving heads, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
In a scanning-head printer such as many ink-jet, dot matrix, and thermal-head printers the image is rendered on the page by multiple passes of the printhead across the page. Each pass results in the printing of a swath of the image. Overall printing speed is often improved by printing bi-directionally, that is, by printing a swath from left to right, advancing the paper, printing the next swath from right to left, advancing the paper, and so on. However, for bi-directional printing, the problem of misregistration tends to occur between swaths due to the backlash from the changed motion of the printhead.
One solution to the problem is given in U.S. Pat. No. 5,044,796 to Mark Lund, assigned to Hewlett-Packard Co. The '796 patent seeks a scan line at which there is a vertical break. If such a scan line is found (and it is greater than 3/4 of the way down the printhead) then instead of imaging the full printhead height of data, the method only images data down to the break. The head is then advanced by the distance to the break (not the printhead height) and the next swath starting from this point is analyzed and rendered. If suitable breaks are found, the printing is done bi-directionally, otherwise uni-directional printing is employed. Thus, the reverse in head direction always occurs along break scans. The break scans are scans in which nothing is printed or in which there is a break in vertical alignment (i.e., for every dot in the scan, there are no dots either directly under it or at a 45-degree angle to it in the next scan). In this manner, since there are no continuous vertical edges across a break scan, it is difficult to detect the effects of misregistration. Generally, this method works, but it is too restrictive in that for bi-directional printing to be enabled, a single scan line has to be a valid break point for all columns of pixels in a given swath. Moreover, bi-directional printing is not always enabled and unidirectional printing results in generally slower rendering.
The present invention contemplates a new and improved bi-directional printing technique that overcomes the above-referenced problems and others.